The rules governing crew size just don't make any sense. Why would you need up to nine people to operate a conventional 23rd century tank?

The machine guns are not used most of the time. One of them is described as coaxial to the main gun. That usually means the tank gunner, one person, has a secondary anti-vehicle/anti-personnel weapon. Likewise the other MG would be the province of the tank commander. The driver doesn't operate weapon systems because he's driving. It makes no sense that control of the LRM's can't be switched manually between the gunner and the tank commander at the discretion of the tank commander.

Why bother having even a simple fire control system if every weapon has to be managed by it's very own passenger?

Overview:
Manufactured by Terra’s biggest producer of armored vehicles, Leopard Armor,
the Merkava Heavy Tank entered service with Terran Alliance heavy armor
platoons in 2294. There, it quickly proved to be a reliable and dependable main
battle tank. The Alliance Global Militia quickly deployed Merkavas across all
of it's commands. Even though it was starting to show it's age by the time of
the Alliance Civil War, the Merkava proved its worth during the battles of
Zurich and Bangkok, outclassing older main battle tanks. After the formation of
the Terran Hegemony, the Merkava remained the keystone of the HAF’s armor
formations. The Merkava would continue to serve, with a series of minor
upgrades, until it was replaced by the Merkava Mk II in 2384. The Merkava line
of tanks would continue to serve in the HAF for almost three centuries.
Capabilities:
The Merkava line of tanks have each followed the same proven formula for that
had become typical by the Age of War; a turret-mounted main gun, supported by
missile launchers and machine guns. For both the original Merkava (subsequently
known as the Mk. I) and the Mk. II, this primary weapon was the dreaded 150 mm
Heavy Cannon. Though not appreciated in modern times, this weapon was capable
of penetrating even the heaviest armor available at the time, potentially
disabling any opposing vehicle with a single direct hit. The Merkava also
carried one co-axial machine gun in the turret and one forward machine gun to
fend off infantry attacks.
The most significant innovation offered by the Merkava were it's recently
developed Harpoon LRM launchers, instead of the unguided rocket launchers of
its progenitors. A leap forward in missile technology, the LRM launchers were
able to feed targeting data from the Merkava's fire control system directly to
the missiles in-flight. Even though unguided rockets of the day could yield
higher burst damage, the longer range and improved accuracy of the LRM launcher
made it a much more reliable weapon. In addition, it featured an auto-loading
mechanism allowing it to fire repeatedly during battle instead of only once.
However, the accuracy of the LRMs suffered greatly at close range. This
combined with its low top speed, made it difficult for the Merkava to fend off
an enemy that managed to get close. As some compensation, 10% of the Merkava's
payload was devoted to armor and it utilized the toughest armor composite
available at the time.
Variants:
Prior to the Alliance Civil War, a few minor upgrades to the Merkava had been
developed but not rolled out consistently across all Militia platoons. The
first was a simple update to the LRM launchers which were more accurate than
the early models. Second was an update to the fire control software that
improved the general accuracy of all the Merkava's weapons. This upgrade was
easy to apply and all Merkava units had received it by the time of the Civil
War. Finally, an upgrade to a more advanced fire control system. This new
system offered even greater accuracy, but required sacrificing the forward
mounted machine gun. In addition, many crews reported random software glitches
with the new system. This update was not widely deployed by the time of the
Civil War. After the formation of the Terran Hegemony, the HAF ensured that the
glitches were addressed and all Merkavas still in service were updated.
In 2384, Leopard Armor would roll out a completely re-engineered upgrade to the
Merkava line. The new tank would be known as the Merkava Mk II, thus forever
designating the orginal Merkava as the Mk I. The Mk II, used improved armor
composites that were lighter and allowed for a much lighter chassis all without
losing any durability. These weight savings allowed for the installation of a
more powerful engine, giving the Mk II a 20% boost to top speed. It also
allowed one of the LRM-5 racks to be replaced with a ten-tube rack, while the
other was replaced with an SRM-4 pack. This combination addressed the lack of
close range firepower without sacrificing long range firepower. The other
weapons were identical to the original Mk I, including the return of the
forward mounted machine gun.
The Mk II would serve until the mid 25th century when it would be displaced by
the Mk III. The Mk III was slower, with a top speed equal to the Mk I, but
would feature an advaned armor composite which could fully dissipate the
destructive force of Heavy Cannons and Laser Cannons.